The present invention relates to a heat sensitive transfer medium for use in a multi-gradation printing process in which the density of a print image can be changed. More particularly, it relates to a multi-gradation heat sensitive transfer medium for use in a multi-gradation thermal transfer process for adjusting the density of a transfer image by superimposing ink layers on a recording medium a plurality of times by thermal transfer.
As a conventional multi-gradation thermal transfer method, there is known a technique commonly called 3 L method (see Japan Denshi Tsushin Gakkai's technical report IE 81-63, p 45 to 52, Sept. 25, 1981).
This 3 L method is intended to obtain a multiplicity of gradations through combinations of dot number variations in a picture element with reflection density variations in thermally transferred ink layers. With this method, however, it has been very difficult to obtain a multiplicity of image gradations close to natural tones and yet having a high degree of resolution, becuase it has a limitation in the adjustment of reflection density of the ink layers and because the number of dots is subject to limitation relative to resolution.
In order to overcome the drawback of the 3 L method, there may be considered a method in which printing is carried out by superimposing ink layers of the same color which have the same density or different densities, thereby adjusting the density of the print image. According to this method, it is possible to use the number of times of superimposing as a means for providing multi-gradation and to improve resolution by decreasing the number of dots per picture element but yet obtain a some number of gradations.
However, such multi-gradation method utilizing superimposing printing technique has a disadvantage that the density of a subsequently transferred ink layer produces a stronger effect than that of a previously transferred ink layer so that it is difficult to obtain a progressive increase in density, thus the print image produced being likely to be of a foggy and light tone even at a maximum reflection density.